US2880151A - Electrolytic production of magnesium metal - Google Patents

Description

flied my 11, 1957 March 31, 1959 gum m, 2,880,151 ELECTROLYTIC kpbvcnou 0 MAGNESIUM- METAL I I 2 Sheets-Sheet 1 mmvr'oxs. L/oyaG. Dean Franc/335k OKs/ uasi: Ken Pose g,Jr. I w M {MA HTTOR/VEYS Y United States Patent ELECTROLYTIC PRODUCTION OF MAGNESIUM METAL Lloyd G. Dean, Lake Jackson, Franciszek OlstoWski,

Freeport, and Ken Posey, Jr., Lake Jackson, Tex., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application February 11, 1957, Serial No. 639,436

2 Claims. (Cl. 204-70) This invention relates to electrolytes for, and methods of, electrolytic production of magnesium. It especially relates to molten salt mixtures as electrolytes having a density less than that of molten magnesium and to the method of producing magnesium therefrom.

Magnesium is currently produced on a large scale by the electrolysis of a fused salt mixture containing magnesium chloride. These electrolytic baths have a density greater than that of molten magnesium. Unless the cathode in the bath consists of a molten metal which alloys with molten magnesium produced by the electrolysis, the liberated magnesium rises to the surface of the bath and is removed therefrom .in recovering the magnesium so produced. The use of a molten cathode with which the produced metal becomes alloyed entails a complex subsequent separatory step to recover the magnesium therefrom and is not economically feasible. In current practice, therefore, the magnesium is permitted to rise to the surface of the electrolyte. Since the gaseous products of the electrolysis are also lighter than the bath, and therefore rise to and evolve from the surface, either elaborate means for preventing recombination of the magnesium with the evolving gaseous products havebeen found necessary when producing strong chlorine or the cell must be swept with some sufiicient-ly unreactive gas, usually air. The :first of these causes the cell to operate at a higher voltage, hence to consume more power, while the latter produces a dilute chlorine and sacrifices some current efficiency.

.A further difliculty encountered with baths heavier than molten magnesium is that the magnesium, as it collects at the surface of the bath, is exposed relatively unprotected from .air which .gives rise to a constant threat of burning of the magnesium, particularly at higher operating temperatures. .As a result, .a significant amount of the liberated magnesium is reoxidized and unrecoverable.

Furthermore, the efficiency of known electrolytic methods of producing magnesium has not been satisfactory as shown -.by the number of kilowatt-hours of power required to produce a pound of magnesium. This low efficiency is due largely to an undesirably wide spacing of electrodes which is made necessary by the relatively high density of the electrolyte employed, to recombination of the products of electrolysis, to oxidation, and to the fact that the character of the electrolyte requires a cell design which inherently results in uneconomical 1 R losses in the cell, electrodes, and external connections.

In view of the diificulties attendant upon conventional methods of electrolytic production of magnesium, it is a desideratum in the art to provide an improved electrolyte for and method of producing magnesium electrolytically.

According to the invention we have provided an improved fused salt electrolyte and method of producing Patented Mar. 31, 1959 earth metal, from 56 to percent of potassium chloride,-

and other chlorides of alkaline earth metals, e.g., calcium chloride and sodium chloride, to the extent that such other chlorides do not result in an electrolyte having a density which is not at least 0.034 gram per cc. less than that of molten magnesium at the temperature of the elec* trolysis. By fluoride fraction is meant that portion of a fluorine-containing salt which would be elemental fluorine on analysis. In electrolyzing the improved electrolyte between a suitable anode and cathode in accordance with the invention, the magnesium is produced out of contact with the atmosphere below the surface of the electrolyte, in a highly efficient and convenient manner, and is recoverable without significant loss. The fluoride has been found to aid the collection of the molten magnesium therein into a coalescent mass or regulus.

A characteristic of our novel electrolyte, and method of producing magnesium therefrom which particularly enhances its utility, is its ability to tolerate water-containing salts in the feed used to replenish the electrolyte dur-' ing electrolysis. For example, water may be present in the magnesium chloride feed, employing our electrolyte, to the extent of 30 percent and may be present in amounts up to 25 percent without lowering the cathode efficiency as much as 5 percent. The reason for the high tolerance of water is not known for certain but the presence of potassium chloride in the electrolyte in the percentage range encompassed by the invention appears to effect a condition in the electrolyte which substantially inhibits the reaction of water with magnesium chloride that the chlorine produced is not excessively diluted by extraneous gases. This objective is attained without the sacrifice of power loss that is encountered by the use of baflles, partitions, or curtains.

A third characteristic of our novel electrolyte, and method of employing it, which further enhances its utility is its low initial cost. Furthermore, although potassium chloride forms the major proportion of the electrolyte, his not substantially decomposed during the electrolysis and only small losses need be made up from time to time.

The invention then consists of the improved electrolyte and method of producing magnesium therefrom herein fully described and particularly pointed out in the claims, reference being made to the accompanying drawing.

In said drawing:

Figure 1 is a plan view of an electrolytic cell with which the invention may be practiced.

Figure 2 is a sectional elevation along line 2-2 of Figure 1.

Figures 3 and 4 are graphs showing the operating density range and corresponding percentages by weight of salts composing the fused salt mixtures of the invention.

Referring now to Figures 1 and 2 of the drawing in detail, there is shown steel shell 1 enclosing refractory brick setting 2. Iron pot 3, having a flanged rim, is placed in the furnace setting. The pot is electrically insulated onthe inside with ceramic lining 4. Electrolyte 5, formulated in accordance with the invention, is placed in lined pot 3. The top of the cell is provided with ceramic-lined metal cover 6. AC. electrodes 7, extending downwardly through openings provided therefor in cover 6, provide a means for passing an AC. through the electrolyte to supply heat to maintain it in a molten state. The A.C. electrodes may be lowered or raised by means of chain falls (not shown) according to the need for the passage of more or less heating current. In cover 6 is opening 8 for admission of feed and access to the pot. Opening 8 is provided with removable cover 9. Outlet 10 is provided for egress of chlorine and other gases, if any, formed during electrolysis. Drain assem- -bly 11, having a valve therein, is provided at the lower portion of pot 3 as an alternative means for removing magnesium metal. Extending through an opening in cover 6 and into pot 3 is anode 12. The degree to which anode 12 extends is controlled by a chain fall (not shown). Steel or graphite cathode 13 is shown at the bottom of pot 3. Current leads 14 and 15 are connected to the anode and cathode respectively by suitable terminals 16 and 17 respectively. Packing gland 18 is positioned about the anode and packing glands 19 are positioned about A.C. electrodes 7 for snug fits with and insulation from the openings provided therefor in cover 6.

In carrying out the invention, the heated pot is charged with electrolyte formulated in accordance with the invention or its separate ingredients. Heat is usually initially applied to the charge by inserting a heating means such as a gas flame through opening 8 and playing it on the charge until it is sufficiently molten to conduct current. The lower ends of the AC. electrodes 7 are dipped into the molten charge and current is then passed between A.C. electrodes 7 until a suitable temperature for electrolysis is reached. If desired, the electrolyte ingredients may be melted in a separate vessel and introduced slowly in the molten condition into the electrolytic cell. When a suitable temperature has been reached, electrolysis is effected by applying a suitable between anode 12 and cathode 13. A suitable temperature for electrolysis employing the novel electrolyte of the invention is in the range of 815 degrees to 900 degrees C. and preferably 825 degrees to 900 degrees C.

As the electrolysis proceeds, the magnesium liberated at the cathode accumulates at the bottom of the cell to form the molten body of magnesium 20. Chlorine is liberated at the anode and rises to the surface of the electrolyte where it is withdrawn from the cell, as through outlet 10. The accumulated molten magnesium may be recovered by means of a dipper or a siphon inserted through opening 8 or by means of the drain assembly 11 by opening the valve therein. When the cell is in operation, a portion of the accumulated molten magnesium 20 may be allowed to remain on the floor of the cell and thus serve as a cathode.

In the electrolysis, as the magnesium chloride in the electrolyte becomes depleted during operation of the cell containing the electrolyte of our invention, it must be replenished either at intervals or continuously to maintain the desired proportion of it in the fused bath. Occasional additions of potassium chloride, and a fluoride if employed, may be necessary to maintain their proper proportions in the electrolyte.

During the electrolysis a small amount of non-metallic insoluble matter called sludge may accumulate at the bottom of the cell. The fluoride, if added as already mentioned, also aids in settling the sludge which stratifies below the molten magnesium in the cell. Such sludge may be removed either by a dipper as in the case of the produced metal or by draining through drain assembly 4 11 by opening the valve therein. Desludging operations are necessary after protracted operation but no more frequently than is now required in current practices.

It is understood that the cell of Figs. 1 and 2 is illustrative of but one form of cell for use in the practice of the invention and that other forms of cells and modifications of that shown may be used with the novel electrolyte according to the method herein described. Generally, however, the cell required for carrying out the invention is comparatively simple in design since no precautions are necessary to prevent recombination of the chlorine and magnesium.

In the preferred embodiment of the invention, the electrolyte includes from 0.1 to 1.0 percent, but preferably from 0.25 to 0.75 percent, by weight of the fluoride fraction, i.e. the elemental F portion, of a fluoride of an alkali or alkaline earth metal. In the case of CaF for example, since F constitutes of CaF the amount of CaF to be added is of 0.25 to 0.75 or between about 0.51 and 1.54 percent.

The magnesium chloride content of the electrolyte is to be at least 5 weight percent because electrolytic dissociation of potassium chloride becomes objectionable at less than 5 percent by weight of magnesium chloride in the electrolyte. On the other hand, the percentage of magnesium chloride is to be no higher than that which will result in a density differential between the electrolyte and the molten magnesium, at operating temperatures, of at least 0.034 g. per cc. which occurs when the magnesium chloride is about 44 weight percent at 850 to 900 C. in the presence of 1 percent CaF and substantially free from other materials. If the density of the electrolyte is not at least 0.034 g. per cc. less than the molten magnesium, the magnesium deposition tends to become erratic due to the tendency of some of the magnesium to remain suspended in the electrolyte.

The cell suffers some loss of efliciency when the magnesium chloride content of the electrolyte is between 30 percent and 44 percent so therefore more eflicient operation is obtained when the magnesium chloride is not over 30 percent. The preferred range for magnesium chloride is from 8 to 28 percent by weight of the fused electrolyte.

Referring to Figures 3 and 4 in detail, lines a and a respectively show the density of the electrolyte of the invention having varying percentages of KCl and MgCland containing 1 percent of Calat 850 and 900 C. respectively. Lines b and b of Figures 3 and 4 respectively show the density of magnesium metal at the same two temperatures respectively. Lines 0 and 0' respectively show the maximum permissible density of the electrolyte, i.e. a density which is 0.034 g. per cc. less than that of magnesium at the temperature of the electrolyte. A and A, the shaded portions, represent, respectively, the density operating areas between 5 percent MgCl and P and P percent MgCl; in which the density of the electrolyte is 0.034 g. per cc. less than that of magnesium metal at the temperatures of 850 and 900 C.

l It is manifest from Figures 3 and 4 that P and P' reprey The temperature of electrolysis was 850 sent the highest percentages of MgCl permitted at 850 and 900 C., respectively. It may be readily observed that much greater tolerance of other chlorides than those of magnesium and potassium as described hereinbefore, e.g., chlorides of sodium and calcium, may be tolerated at percentages of MgCl approaching the 5 percent minimum requirement, than those approaching P and P percent of MgCl Tables I and II set out the operating conditions and pertinent data obtained in a series of examples of the practice of this invention. Anhydrous feed was used in the examples of Table I and hydrous feed, containing 25 percent water, was used in the examples of Table II. C. The density difierential between the electrolytes and molten magnesium in the examples set out in the tables and 0.054 gram per cc.

is between 0.052v

TABLE I Examples m which anhydrous MgCl feed used Anode Avg. Comp. of Electrolyte, Wt. Cathode Anode Oath Kwh./ Length Average to Oathpercent Curr. Curr. Curr. 1b. Mg Examples 01 Run, Voltode Den., Den., E11, Pro- Hours age 1 Spacing Amps./ Amps] Perduced 5 (ln.) K01 MgGh CaF, MgO in? cent 3 7. 5. 88. 2 11. 7 none 0.05 79. 4 6.9 18.5 5. 85 3 79. 3 19. 7 0. 51 0.01 3.6 3. 3 74.0 7. 9 18. 1 5. 85 3 81.1 18.1 0.33 0.08 3. 2 3.0 90.0 6. 5 17. 6 5. 0 1. 5 79. 1 18. 6 0. 39 0. 03 2. 3 2.1 90.4 5. 5 18.0 5. 3 1. 5 79. 3 20.5 0. 93 2. 9 2. 7 88. 8 6. 0 43. 0 5. 2 1. 5 80. 4 18. 9 0. 58 0 3. 1 2.9 75.0 6. 9 165. 5 5. 8 1. 5-3 80.0 29. 0 0.43 3.0 2. 8 77. 2 7. 5 22. 2 5. 4 3 79. 2 19. 2 0. 58 2. 2 2. 0 82. 2 6. 5 17. 4- 5.9 3 79.3 19.3 0.45 3.0 2. 8 91. 3 6. 5 16.2 4. 9 3 80.2 18.2 0.61 2. 7 2. 5 90. 6 5. 4 15. 3 4. 6 3 80. 6 17.9 0. 46 2. 6 2. 4 91. 0 5. 0 14. 0 4. 9 3 82. 5 16.1 0. 38 2. 8 2. 5 92. 2 5. 3 19. 0 4.9 3 80.8 17. 5 0. 67 2. 7 2. 5 80.0 6. 1 17.8 4. 8 3 86. 7 11. 6 0.70 2. 4 2. 2 73. 2 6. 6 18.3 5. 3 3 89. 6 8. 4 1.00 0. 03 2. 6 2. 4 86.0 6. 2 19. 28 6. 8 3 79. 0 20. 0 1. 00 1. 6 5. 3 83. 7 8. 1 430. 00 6. 2 1. 5 79. 0 20. 0 1. 00 9. 3 91.1 6. 8

TABLE 11 Examples m which MgCl hydrate 6 used Elec- Avg. Comp. of Electrolyte, Cathode Anode Cathode Length Avg. trode Weight percent Curr. Curr. Curr. Kwh./ Examples of Run, Volts 1 Spacing, Den., Den., E11, 1b. Mg 5 Hours Inches AmpsJ Amps] percent 3 K01 MgCla Cali: MgO mi in.

1 Voltage applied between anode and cathode. 2 Calculated on total immersed area. (1 d wt. of Mg Pro uce X100 percent Cathode current efificlency wt. of Mg Possible according to Faradays Law.

4 Not determined.

5 Kwh./lb. Mg was calculated according to the formula: 6 Average percent water in feed= percent.

TYPICAL ANALYSIS OF GAS EVOLVED FOR WHICH ANALYSIS WAS MADE IN EXAM- PLES OF TABLE II Ch H01 0 0: 03 N; C O

Referring to the tables above, it can be seen that high efliciency and low power consumption in an electrolytic cell employing the novel electrolyte are obtainable by the invention. Examples 4, 5, 9, 10, 11, 12 and 17 show cathode efficiencies of at least 90 percent and power consumption as low as 5.0 and not over 6.8 kw. h. per pound of magnesium produced. The examples shown in the tables were run at anode current densities varying between 2.0 and 9.3 amperes per square inch of immersed area and at anode-to-cathode spacings of 1.5" and 3". The results show that the eflect of such variations in current density and spacing, as well as the presence of magnesium oxide and water to the extent shown, is not appreciably detrimental. Although no coalescent agent is essential, as shown by Example 1, much better coalescence of the magnesium occurred in the examples in which CaF was present as a coalescent agent; the collection of the magnesium into a body and its removal from the cell were facilitated by the presence of the CaF The fluoride was conveniently added as fluorite or fluorspar. Example 1 also further shows that magnesium oxide may be tolerated whether or not a fluoride is also present.

No serious difliculties were encountered in operating applied voltageXamp.Xhours 100011). of Mg produced Mg Pmducedof the cell at 900 C. This fact is an unexpected discovery. Heretofore, temperatures above 800 C. have been thought detrimental to the economical production of magnesium metal by electrolysis of molten salts containing magnesium chloride. This erroneous thought associated with known electrolytes was likely based on opinions concerning metal fogging in the region of the cathode, high vapor pressures of the salt components and molten magnesium, and increased burning of magnesium metal at the surface of the electrolyte. These detrimental effects are not encountered in the present method.

Impurities up to 1 or 2 percent by weight normally present in the feed, such as magnesium oxide, and traces of salts and oxides of other metals among which are those of iron, copper, nickel, silicon, manganese, lead, titanium, boron, aluminum, and chromium, may be tolerated. Since most of these metals deposit at a lower potential than does magensium, the specifications of the magnesium to be produced will predetermine the permissible amounts of such metals.

Among the advantages of the invention are: production of relatively pure magnesium metal below the surface of the electrolyte; production of chlorine in a concentrated form as a by-product; comparatively simple cell requirements; high-temperature operation Without adverse efiects; negligible loss of metallic magnesium; high elficiency as reflected by low power consumption per pound of magnesium produced; wide tolerance of Water in the feed; low initial cost of the salt in which electrolysis takes place; employment of an alkali or alkaline earth fluoride, the presence of which readily coalesces the molten magnesium and facilitates its separation from the electrolyte and sludge.

Having thus described our invention, What we claim and desire to protect by Letters Patent is:

1. In the method of producing magnesium metal and chlorine gas of high purity by electrolyzing a molten salt bath comprising KCl and MgCl which is of less density than the molten magnesium at between 815 and 900 C. the improvement which consists of adding to the bath during electrolysis hydrous MgCl feed containing between 0.5 and 4.0 moles of H 0 per mole of MgCl to maintain the MgCl content of the bath between 5 and 30 percent by weight and to maintain the density of the bath at not less than 0.034 gram per cubic centimeter less than that of molten magnesium at the temperature of electrolysis.

2. In the improved method according to claim 1, the step of adding MgF to the bath in suflicient amount to 8. provide up to 1 percent by weight of the fluoride fraction thereof based on the weight of the bath.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Ser. No. 340,402 (A.P.C.), published May 18, 1943.

Claims (1)

1. IN THE METHOD OF PRODUCING MAGNESIUM METAL AND CHLORINE GAS OF HIGH PURITY BY ELECTROLYZING A MOLTEN SALT BATH COMPRISING KCI AND MGCL2 WHICH IS OF LESS DENSITY THEN THE MOLTEN MAGNESIUM AT BETWEEN 815* AND 900*C. THE IMPROVEMENT WHICH CONSISTS OF ADDING TO THE BATH DURING ELECTROLYSIS HYDROUS MGCL2 FEED CONTAINING BETWEEN 0.5 AND 4.0 MOLES OF H2O PER MOLE OF MGCL2 TO OBTAIN THE MGCL2 CONTENT OF THE BATH BETWEEN 5 AND 30 PERCENT BY WEIGHT AND TO MAINTAIN THE DENSITY OF THE BATH AT NOT LESS THAN 0.034 GRAM PER CUBIC CENTIMER LESS THAN THAT OF MOLTEN MAGNESIUM AT THE TEMPERATURE OF ELECTROLYSIS.

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